1. The Present Development
The present development is a process for converting aqueous sorbitol to xylitol and isosorbide in the presence of an acid catalyst. Specifically, the process involves reacting an aqueous sorbitol solution with an acid zeolite at a temperature of about 250° C. in a reactor maintained at from about 68 bar to about 80 bar pressure to produce xylitol and isosorbide in high yields.
2. The Prior Art
Xylitol is well known for its extensive uses in the food industry. For example, xylitol is commonly found in diet drinks, chewing gums, ice cream, baked goods and fruit spreads. The global market for xylitol, based on data in 2000, was estimated at 0.97 billion, with sales volumes of 1,397,000 metric tons.
Isosorbide, or 1,4-3,6-dianhydro-D-sorbitol, is known for its therapeutic uses, particularly in the form of mono- or di-nitrate compounds. Isosorbide has also been shown to have utility as a monomer in the manufacture of polymers and copolymers, especially polyester polymers and copolymers. Polymers made of isosorbide tend to exhibit high clarity and mechanical strength.
A common method for production of xylitol is through a multi-step process involving hydrolysis of a xylan-containing material to produce xylose, then conversion of the xylose to xylitol, generally in the presence of a nickel catalyst, such as Raney nickel. Though this method results in the desired conversion, the method tends to be expensive and inefficient because the resulting xylitol must be separated from numerous by-products of the reaction. Alternatively, xylitol may be produced by enzymatic conversion of sugars. However, these methods have not been demonstrated to be effective on a commercial scale.
A common method for production of isosorbide is to contact sorbitol with an acidic catalyst, such as a sulfuric acid catalyst or an acid zeolite, followed by separation and purification. This method results in the desired conversion, but tends to be expensive and difficult to maintain on a commercial scale because of the need for maintaining high vacuum during the reaction. Alternatively, isosorbide may be produced by reacting an aqueous solution of sorbitol with an acid catalyst, including acidic zeolites, and an acid-stable hydrogenating catalyst, at a temperature of greater than 100° C. in a hydrogen atmosphere. However, this method tends to produce a mixture of products that require further separation and purification.